drm/radeon: disable tests/benchmarks if accel is disabled

commit 4a1132a023eb48cf10522d84c5908d43b612c041 upstream.

The tests are only usable if the acceleration engines have
been successfully initialized.

Based on an initial patch from: Alex Ivanov <gnidorah@p0n4ik.tk>

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>